W149 Datasheet, PDF (3/14 Pages) Cypress Semiconductor – 440BX AGPset Spread Spectrum Frequency Synthesizer

W149
Overview
The W149 was developed as a single chip device to meet the
clocking needs of the Intel 440BX AGPset. In addition to the
typical outputs provided by standard 100-MHz 440BX AGPset
FTGs, the W149 adds a thirteen output buffer, supporting
SDRAM DIMM modules in conjunction with the chipset.
Cypress proprietary spread spectrum frequency synthesis
technique is a feature of the CPU and PCI outputs. This feature
reduces the peak EMI measurements of not only the output
signals and their harmonics, but also of any other clock signals
that are properly synchronized to them.
Functional Description
I/O Pin Operation
Pins 7, 25, 26, 46 are dual-purpose l/O pins. Upon power-up
these pins act as logic inputs, allowing the determination of
assigned device functions. A short time after power-up, the
logic state of each pin is latched and the pins become clock
outputs. This feature reduces device pin count by combining
clock outputs with input select pins.
An external 10-kΩ “strapping” resistor is connected between
the l/O pin and ground or V_DD. Connection to ground sets a
latch to “0”, connection to V_DD sets a latch to “1”. Figure 1 and
Figure 2 show two suggested methods for strapping resistor
connections.
Upon W149 power-up, the first 2 ms of operation is used for
input logic selection. During this period, the four I/O pins (7,
25, 26, 46) are three-stated, allowing the output strapping re-
sistor on the l/O pins to pull each pin and its associated capac-
itive clock load to either a logic HIGH or LOW state. At the end
of the 2-ms period, the established logic “0” or “1” condition of
the l/O pin is latched. Next the output buffer is enabled, con-
verting the l/O pins into operating clock outputs. The 2-ms tim-
er starts when V_DD reaches 2.0V. The input bits can only be
reset by turning V_DD off and then back on again.
It should be noted that the strapping resistors have no signifi-
cant effect on clock output signal integrity. The drive imped-
ance of clock output is <40Ω (nominal), which is minimally af-
fected by the 10-kΩ strap to ground or V_DD. As with the series
termination resistor, the output strapping resistor should be
placed as close to the l/O pin as possible in order to keep the
interconnecting trace short. The trace from the resistor to
ground or V_DD should be kept less than two inches in length to
prevent system noise coupling during input logic sampling.
When the clock outputs are enabled following the 2-ms input
period, the specified output frequency is delivered on the pin,
assuming that V_DD has stabilized. If V_DD has not yet reached
full value, output frequency initially may be below target but will
increase to target once V_DD voltage has stabilized. In either
case, a short output clock cycle may be produced from the
CPU clock outputs when the outputs are enabled.
W149
Power-on
Reset
Timer
V_DD
Output
Buffer
Output Three-state Hold
Output
Low
QD
Data
Latch
10 kΩ
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10 kΩ
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Clock Load
Figure 1. Input Logic Selection Through Resistor Load Option
Jumper Options
W149
Power-on
Reset
Timer
Output
Buffer
Output Three-state Hold
Output
Low
QD
Data
Latch
10 kΩ
V_DD
Output Strapping Resistor
Series Termination Resistor
R
Clock Load
Resistor Value R
Figure 2. Input Logic Selection Through Jumper Option
3